1. Field of the Invention
The present invention relates to a process of producing a vinyl chloride paste.
2. Description of the Prior Art
It is known that by using vinyl chloride paste resins in the form of small particles (that is, about 0.1 - 2 microns) relatively stable dispersions of the paste resin in a plasticizer may be formed, which dispersions are gellable on heating to yield a protective coating or articles.
In the case of preparing a vinyl chloride resin sol by blending a plasticizer, a stabilizer, etc., with a vinyl chloride paste resin and mixing by stirring, if the mixture contains foam, the interior and the surface appearance of the final product are degraded, which results in reducing the commercial value of the products.
If the thermal stability of the plastisol is poor, dechlorination occurs upon gelatinization and melting of the plastisol thereby deteriorating the physical properties of the resulting articles.
Further, the plastisol must generally have a initial viscosity as low as less than 8000 cps and be stable. Generally, 1 week after preparation of the sol, the sol viscosity must be less than 10,000 cps.
Furthermore, the mechanical properties of the products are reduced, and hence the mixture is usually subjected to an air release procedure or "defoaming" under vacuo during kneading or after kneading. However, such an air release operation requires great effort and long periods of time, which greatly reduces the workability of prior art processes.
A vinyl chloride paste resin has usually been prepared by an emulsion polymerization method using a water-soluble catalyst or polymerizing vinyl chloride by homogenizing the reaction mixture using an oil-soluble catalyst. In the polymerization, a anionic surface active agent such as an alkali metal salt or an ammonium salt of a higher alcohol sulfonic acid ester or an alkylbenzene sulfonate has been used as an emulsifying agent.
The vinyl chloride polymer latex prepared by such an emulsion polymerization is dried by means of spray drying and the like, but to provide mechanical stability to the latex at drying or to reduce the viscosity of the sol prepared from the vinyl chloride resin, a proper amount of the aforesaid anionic surface active agent or a nonionic surface active agent such as a polyoxyethylene alkyl ether, polyoxyethylene-polyoxypropylene block copolymer, sorbitan ester, glycerine alkyl ester, etc., is sometimes added.
However, in the conventional production processes using the above-described emulsifying agent for polymerization or preparation, the air release capability of the sol is poor, and hence such a paste resin is of inferior quality.